One method magnetic tape devices utilize to maximize capacity is to maximize the number of parallel tracks on the tape. The typical way of maximizing the number of tracks is to employ servo systems which provide track following and allow the tracks to be spaced very closely. Even so called "low end" tape devices are now employing track following to maximize the number of tracks.
An example of track following servoing is the provision of groups of prerecorded parallel longitudinal servo tracks that lie between groups of longitudinal data tracks, so that one or more servo heads may read the servo information and an accompanying track following servo will adjust the lateral position of the head or of the tape to maintain the servo heads centered over the corresponding servo tracks. The servo heads are spaced a defined distance from the data heads, so that centering the servo heads results in the data heads being centered over the data tracks. The defined distance is maintained for all tape drives in a particular family allowing exchange of tape media between tape drives in the same or compatible families.
An example of a track following servo system particularly adapted to tape comprises that of the Albrecht, et al. U.S. Pat. No. 5,689,384. The servo patterns are comprised of magnetic flux transitions recorded in continuous lengths across a track at non-parallel angles, such that the timing of the servo transitions read from the servo pattern at any point on the pattern varies continuously as the head is moved across the width of the servo pattern. For example, the pattern may comprise a pair of opposed transitions, each called a "chevron", which appear as a diamond, which transitions are sloped or slanted with respect to the track in the longitudinal direction. Thus, the relative timing of transitions read by a servo read head varies linearly depending on the lateral position of the head. Speed invariance is provided by utilizing a group of interlaced pairs of transitions and determining the ratio of two timing intervals, the interval between two like transitions compared to the interval between two dissimilar transitions. Synchronization of the decoder to the servo pattern may be accomplished by having two separate groups of pairs of transitions, each group having a different number of pairs of transitions. Thus, the position in the set of groups is readily determined by knowing the number of pairs of transitions in the present group.
In order to determine the longitudinal position of the tape, the Albrecht et al. U.S. Pat. No. 08/859,830 application discloses a magnetic tape media having data information superimposed on the timing-based servo information of the Albrecht et al. U.S. Pat. No. 5,689,384, which data information may comprise longitudinal addressing or tachometer information. At least two transitions of the servo information are shifted longitudinally with respect to other of the transitions of the servo information, the shifted transitions comprising the superimposed addressing data information. The reason two transitions are shifted is to maintain the timing between pairs of transitions required for the timing-based servo. The two shifted transitions may be in the same set of patterns or "chevrons", or may be in opposed chevrons called a "diamond" pattern.
The codes and resultant patterns described in the Albrecht et al. U.S. Pat. No. 08/859,830 application are primarily adapted for other than the very low end tape drives to contain a large amount of data and are therefore somewhat complicated and require extensive logic to decode.
Such an ability to superimpose data in a timing-based servo is also of high value in very low end tape drives, so as to eliminate the need for an expensive tachometer. The ideal solution required is a coding scheme which allows decoding to be accomplished without extensive use of logic in addition to that of the servo.